Lawful Interception (LI) allows Law Enforcement Agencies (LEAs) to obtain communication network data for the purpose of analysis or gathering evidence. The data typically includes details of signalling, such as called and calling parties, and in some instances the contents of the call itself. 3GPP TS 33.107 “Lawful interception architecture and functions” describes the architecture and functional requirements with a Third Generation Mobile Communication System. FIG. 1 shows the architecture. A Law Enforcement Monitoring Facility (LEMF) 1 may be connected to a 3G network or any other network. An Administration Function (ADMF) 2 communicates with the LEMF 1. Note that more than one LEMF is shown because the ADMF may communicate with several different LEMFs. Owing to different legal LI requirements, the LI information shared with different LEMFs may be different. For simplicity, the following discussing refers to a single LEMF 1. The ADMF 2 communicates with the LEMF 1 using a Mediation Function (MF) 3 via a HI1 interface.
Two Delivery Functions (DFs) are provided. DF2 4 communicates with the LEMF 1 via a HI2 interface and is used to send Intercept Related Information (IRI) to the LEMF 1 using a MF 5. DF2 4 receives IRI from the network via an X2 interface. In a Circuit Switched (CS) network, IRI is triggered by events that may be call-related or non-call related. In a Packet Switched (PS) network, IRI may be triggered by events that are session related or session unrelated.
DF3 6 receives the content of a communication subject to LI and sends this to the LEMF 1 using a MF 7 via a HI3 interface.
The ADMF 2, DF2 4 and DF3 6 communicate with a traffic node, also termed an Intercepting Control Element (ICE) 8. The form of the ICE depends on the network in which it is located. For example, an ICE in an IMS network could be a Proxy-Call Session Control Function (P-CSCF) or a Serving-Call Session Control Function (S-CSCF). It may be a Mobile Switching Centre (MSC) server in a 3G network, a Serving GPRS Support Node (SGSN), a Gateway GSN (GGSN), or a Media Gateway (MGW). An ICE performs interception, and in the event that there is more than one ICE, each ICE performs interception independently of other ICEs. Interception actions that the ICE performs include interrogation, activation, deactivation and invocation. An ICE is sometimes referred to as an Intercept Access Point (IAP). Depending on the types of network involved, an ICE can control interception in different networks, e.g. 2G/3G PS domain, CS domain, SAE/LTE.
New and more advanced technologies are introduced in order to provide the end-user with a better experience and to more efficiently utilize radio resources. As network technologies change, existing and replacement technologies exist in parallel for a time, and session and call continuity between the existing and new technologies is needed for an acceptable end-user experience. However, where session data (such as a call) must be intercepted, LEAs also require continuity of interception for the same session/call.
An example of an existing network being replaced by a new network is a circuit switched (CS) network being replaced by a Long Term Evolution (LTE) network. LTE capable networks do not have embedded CS technology, and so voice and video services are provided to the a user by means of IP Multimedia Subsystem (IMS) services, as defined by e.g. the Voice over LTE (VoLTE) profile adopted by the GSMA. However, in the first phases of its deployment, it is expected that the next generation of packet switched LTE access networks (also referred to as E-UTRAN), will not provide full coverage. As an example, depending on the Network Operator migration strategy, initial installations may focus on the most densely populated areas. Moreover, migration (moving from an existing to a new technology) scenarios may lead to islands of E-UTRAN coverage bounded by either GERAN (GSM access network) or UTRAN (WCDMA access network).
Currently, an LTE subscriber can use a dual mode terminal to initiate calls in the E-UTRAN domain or in the GERAN domain or in the UTRAN domain. To allow a smooth migration towards LTE network, Single Radio Voice Call Continuity (SRVCC) provides the ability to allow transition of a voice call from an LTE packet domain to a legacy circuit domain (GSM/WCMA) when LTE coverage becomes poor during a call which was started as VoLTE/IMS call.
For an operator with a legacy (GSM/WCDMA) cellular network who wishes to deploy IMS/VoIP-based voice services in conjunction with the rollout of an LTE network, SRVCC allows voice call continuity (VCC) in case of inter-domain handover from E-UTRAN to UTRAN/GERAN due to e.g. limited E-UTRAN coverage.
Referring to FIG. 2, 3GPP TS 23.216 “Single Radio Voice Call Continuity (SRVCC)” provides that a procedure for single radio voice continuity (SRVCC) is triggered by E-UTRAN towards a Mobility Management Entity (MME), based on measurement reports S1 received by a User Equipment (UE) 9 from a E-UTRAN access network 10. The E-UTRAN access network 10 informs S2 an MME 11 that handover is required, and the MME 11 initiates S3 the SRVCC procedure with the MSC Server 12 enhanced for SRVCC via the Sv reference for a single voice call. The MME 11 also handles S4 a handover for non-voice, if required. The MSC Server 12 enhanced for SRVCC then initiates S5, S6 the session transfer procedure to IMS 14 and coordinates it with the CS handover procedure to the target cell 13. The MSC Server 12 enhanced for SRVCC then sends S7 a PS-CS handover Response towards the MME 11, which includes the necessary CS HO information for the UE to access the UTRAN/GERAN. This is sent S8 to the E-UTRAN cell 10, which sends S10 a handover command to the UE 9, allowing the UE 10 to execute S10 the handover.
Centralization of IMS services and service continuity is specified in 3GPP TS 23.237 and 3GPP TS 23.292. These documents specify that a user must receive services in a consistent manner whether the user accesses the IMS via a CS or a PS. Service continuity is supported between the CS and PS domains. IMS Service Continuity allows for continuing ongoing communication sessions with multiple media formats across different access networks. This level of continuity is required because UEs with multimedia capabilities may move across a multiplicity of different access networks.
Referring to FIG. 3, IMS Service Continuity requires a Service Centralization and Continuity (SCC) AS 15 in the IMS network 14, and a UE 9 with SC capabilities. The SCC AS 15 provides IMS-based mechanisms for enabling service continuity of multimedia sessions. In order to enhance service experience during the inter domain handover, two additional network nodes are provided. An Access Transfer Control Function (ATCF) 16 located between a Proxy-Call Session Control Function (P-CSCF) 17 and a Serving-Call Session Control Function (S-CSCF) 18 and is used as a signalling anchor point. An Access Transfer Gateway (ATGW) 19 is located in the IMS network 14 between the UE 9 and a MSS or MSC 20, and is used for media anchoring.
The ATCF 16 is included in the session control plane for the duration of a session call before and after Access Transfer. During registration, the ATCF 16 provides its address in a header which is later used by the MSC 20 to find the ATCF 16. Signalling on the 12 interface between the ATCF 16 and the MSC 20 relates to an already established session, or a session being established by SRVCC procedures. Correlation between sessions is performed using a C-MSISDN provided to the ATCF 16 during registration.
The ATGW 19 is controlled by the ATCF 16 and stays in the session media path both for the duration of the session and also after Access Transfer, based on the local policy of the serving network.
FIG. 4 shows the signalling and bearer paths for a CS session that allows a subsequent CS to PS transfer when the media path is established using a CS access network. The media path 21 goes directly via the ATGW 19. The signalling path in the access leg 22 comprises standard IMS signalling between the MSC Server 20 and the ATCF 16. The access leg of the signalling path 23 goes to a CSCF 18 via the SCC-AS 15, and the remote leg 24 of the signalling path traverses the Telephony Application Server (TAS) 16.
A Voice over LTE call can be subject to LI for both media and signalling. However, when the VoLTE call is handed-over to GSM/WCDMA (in an SRVCC procedure), in order to provide LI, interception must start in the CS domain. In this case, correlation for interception between and after the hand-over cannot be provided, and so LI continuity cannot be provided when a VoLTE call is subject to inter-domain (to GSM/WCDMA) handover (SRVCC). Lack of correlation between and after the handover could be an issue from a regulatory point of view.